Single-function electronic devices such as resistors and capacitors in the form of chips having electrodes formed at both ends of a body are coming to be used. The reflow method is employed to solder surface mounted devices (SMD) like these chip devices to a printed circuit board. According to the reflow method, a solder paste comprising a solder alloy powder and a paste-like flux is applied by printing or dispensing the paste onto the portions of a printed circuit board to be soldered, i.e., to locations corresponding to the electrodes of SMD's, and then the printed circuit board is heated in a reflow furnace to melt the solder alloy powder to solder the surface mounted devices to the printed circuit board.
In this reflow method, in order to prevent splashing of the paste-like flux during heating as well as to reduce thermal damage to electronic devices and printed circuit boards, preheating is carried out at 100-150° C., and then main heating is carried out to melt the solder alloy powder in the solder paste and to adhere it to the portions to be soldered. In order to reduce thermal damage to electronic devices, the maximum temperature, which is called the peak temperature, is made as low as possible, and the heating time at that temperature is made as short as possible during the main heating.
The main heating temperature is suitably determined based on factors such as the size and thickness of the printed circuit board and the mounting density of electronic parts. Of course, in order to completely melt the solder alloy powder used in the solder paste, the main heating temperature must be higher than the liquidus temperature of the solder alloy powder. Accordingly, the lower the liquidus temperature of the solder alloy powder, the lower the main heating temperature becomes, resulting in less thermal effects on electronic devices. In general, the main heating temperature is said to be 20 to 40° C. higher than the liquidus temperature of the solder alloy of the solder paste.
Recently, the use of lead-free solders which do not contain any lead is being promoted. Lead-free solder alloys are also coming to be used in solder paste.
A lead-free solder is a solder alloy having Sn as a main component to which Ag, Cu, Bi, Sb, Zn, or the like is suitably added in accordance with the use.
In the case of Sn—Cu base lead-free solder alloys, the eutectic composition alloy, i.e., Sn-0.7Cu alloy has a melting point of 227° C. The main heating temperature, therefore, becomes high, resulting in thermal damage to electronic devices during reflow. Moreover, this solder alloy has the problem that its solderability is not good.
In the case of Sn—Bi base lead-free solder alloys, the eutectic composition alloy, i.e., Sn-57Bi alloy has as a melting temperature as low as 139° C., so the main heating temperature is even lower than that of a conventional Sn—Pb eutectic solder, and there is no concern whatsoever of thermal damage to electronic devices during reflow. However, a lead-free solder with this composition contains a large amount of Bi, so it has the property of being extremely brittle, and it has the problem that debonding readily takes place if even somewhat of an impact is applied to soldered portions after soldering.
In the case of Sn—Zn base lead-free solder alloys, the eutectic composition alloy, i.e., Sn-9Zn alloy has a melting point of 199° C., and the main heating temperature is at most 230° C., so there is little thermal damage at the time of reflow. However, Zn has the disadvantages that it easily oxidizes and that its wettability is extremely poor, so it is necessary to perform reflow in a non-oxidizing atmosphere or to use a special flux.
Sn—Ag base lead-free solder alloys have good wettability, so they are already much used. In particular, a Sn—Ag—Cu lead-free solder alloy in which at most 1% of Cu is added to a Sn—Ag solder alloy has better wettability than a Sn—Ag solder alloy, and the strength of the solder alloy is high, so at present, it is the most widely utilized.
However, in the case of Sn—Ag base lead-free solder alloys, the melting point of the eutectic composition alloy, i.e., Sn-3.5Ag, is 220° C., so the main heating temperature becomes at least 250° C., and thermal damage ends up being imparted to electronic devices which are sensitive to heat.
In contrast, a Sn—Ag—Cu lead-free solder alloy has a melting temperature of around 218° C., so the temperature to which a reflow furnace is set for main heating is often around 240° C. Damage due to heat seldom occurs with respect to most SMD's, even when the main heating temperature is around 240° C., but heat-sensitive devices such as semiconductor devices, connectors, and electrolytic capacitors are easily damaged by heat, and they may malfunction.
A lead-free solder alloy has been proposed in which the melting temperature thereof is lowered by the addition of an element such as Bi or In to a Sn—Ag solder alloy or a Sn—Ag—Cu solder alloy. However, since the addition of Bi may decrease the strength of the resulting solder alloy, Sn—Ag—In base solder alloys are widely used for soldering of electronic devices which are less resistant to heat.
A Sn—Ag—In base lead-free solder alloy is a lead-free solder alloy consisting of Sn, Ag, and In, or a Sn—Ag—In solder alloy further containing an element such as Bi or Cu.
As electronic equipment becomes miniaturized, electronic devices are also being miniaturized, as exemplified by 1608 components (16 mm×8 mm) and 1005 components (10 mm×5 mm). When reflow soldering of these small electronic devices is performed using solder paste, because electronic devices are light, it is easy for chips to stand up or tilt during reflow. Standing up or tilting of chips during reflow is referred to as tombstoning or the Manhattan phenomenon.
The phenomenon of chips standing up is a phenomenon in which when a substrate which is printed with solder paste is heated in a reflow furnace, due to a time difference in the heating of the solder paste at both ends of a chip, a time difference develops in the melting of the solder paste at both ends, a moment develops which pulls on one side of the chip, and the chip floats. If the moment which is generated becomes large, the chip ends up standing completely upside down.
Since the phenomenon of chips standing up becomes more prominent as the moment pulling on one end of a chip increases, it occurs more easily with alloys called eutectic composition alloy solders such as Sn-37Pb solder or Sn-3.5Ag in which there is no difference between the solidus temperature and the liquidus temperature. In contrast, in the case of solder alloys such as Sn-2Ag-36Pb (solidus temperature of 178° C. and liquidus temperature of 210° C.), Sn-8Bi-46Pb (159° C. and 193° C., respectively), and Sn-1Ag-0.5Cu (217° C. and 227° C., respectively), the solidus temperature and the liquidus temperature are spaced from each other, and melting of the solder alloy takes place so gradually that the moment pulling on one side of a chip component is lessened, and it becomes difficult for the phenomenon of chips standing up to occur. A Sn-3Ag-0.5Cu lead-free solder alloy, which is currently the most widely used lead-free solder alloy, has a solidus temperature of 217° C. and a liquidus temperature of 220° C., and a small temperature difference exists between them. Therefore, compared to a tin-lead eutectic solder alloy having a composition of Sn-37Pb, the phenomenon of chips standing up during reflow is decreased.
However, the phenomenon of chips standing up is particularly striking with the above-described Sn—Ag—In base lead-free solder alloy. Its advantage that it causes little thermal damage to electronic devices is not being fully utilized.
In the past, as a method of preventing chips from standing up during reflow, it has been proposed to use a solder alloy which can exhibit the twin-peak phenomenon (Japanese Published Unexamined Patent Application 2001-58286).
Mixing of solder alloy powders having at least two different alloy compositions has been carried out in the past. With respect to lead-free solder alloys as well, it is known to improve wettability by mixing a Sn—Zn base powder and a Sn—Zn—Bi base powder (Japanese Published Unexamined Patent Application Hei 9-277082), or to prevent the occurrence of voids or dewetting by mixing a Sn—Bi base powder and a Sn—Zn base powder (Japanese Published Unexamined Patent Application 2002-113590).